Wide frequency offset correction using encoded interburst phase differences

ABSTRACT

Systems and methods for wide frequency offset synchronization are provided. A synchronization data sequence is encoded onto a series of OFDM frequency domain bursts as interburst phase differences between training symbols included within the successive bursts. The interburst phase differences may also encode system configuration information. This technique may be used in conjunction with other synchronization techniques to greatly extend the frequency acquisition range achievable with low cost analog components.

STATEMENT OF RELATED APPLICATIONS

The present application is related to the subject matter of thefollowing four U.S. patent applications:

U.S. patent application Ser. No. 09/245,168, filed on Feb. 5, 1999,entitled SYNCHRONIZATION IN OFDM SYSTEMS.

U.S. patent application Ser. No. 09/244,754, filed on Feb. 5, 1999,entitled ENHANCED SYNCHRONIZATION BURST FOR OFDM SYSTEMS.

U.S. patent application Ser. No. 09/469,715, filed on Dec. 21, 1999,entitled WIDE RANGE FREQUENCY OFFSET ESTIMATION IN OFDM SYSTEMS.

U.S. patent application Ser. No. 09/415,014, filed on Oct. 7, 1999,entitled TRANSMISSION OF SYSTEM CONFIGURATION INFORMATION.

These related patent applications are incorporated herein by referencein their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to digital communications and moreparticularly to synchronization of frequency between a receiver and atransmitter.

In an OFDM (Orthogonal Frequency Division Multiplexing) communicationsystem, a channel to be used for communication is divided intosubchannels that are orthogonal to one another in the frequency domain.Data is communicated in a series of time domain bursts. To form eachtime domain burst an IFFT is applied to a group of frequency domainsymbols and a cyclic prefix is added to the transform result prior totransmission. Transmission may involve conversion of the transformresult to an analog signal, conversion of the analog signal to anintermediate frequency (IF), then upconversion to a desired selectablecarrier frequency prior to final amplification and propagation across atransmission medium. Upconversion is typically achieved by mixing the IFsignal with a variable frequency oscillator signal. The carrierfrequency is varied by varying the oscillator frequency.

On the receiver end, preamplification is followed by downconversion toIF from the carrier frequency, again by mixing with the output of avariable frequency oscillator. The resulting IF signal is typicallyconverted to a baseband digital symbol sequence. The cyclic prefix isremoved and an FFT is applied to recover the original frequency domainsymbols.

For successful communication, the transmitter and receiver should have aprecise shared understanding of the transmission frequency. In theexemplary system described above, this means that the variable frequencyoscillators of the transmitter and receiver should be locked to eachother. Imprecision with respect to the transmission frequency will causeinaccurate recovery of the OFDM symbols. To maintain system performance,it is desirable to always maintain frequency offset between atransmitter and a receiver to within 1% of the spectral width occupiedby a single frequency domain OFDM symbol. When the receiver initiallyacquires the transmitter frequency, it is desirable that thesynchronization system tolerates and corrects as wide as possible afrequency offset between the transmitter and receiver oscillators. Thisallows the use of much lower cost analog components for the receiveroscillator.

U.S. patent application Ser. No. 09/245,168, filed on Feb. 5, 1999, andentitled SYNCHRONIZATION IN OFDM SYSTEMS discloses various systems andmethods for synchronizing the receiver frequency of an OFDM receiver tothe transmission frequency of an OFDM transmitter. One such systemprovides a supplemental cyclic prefix that follows the cyclic prefixused to orthogonalize the frequency domain subchannels. At the receiverend, this supplemental cyclic prefix is correlated to the correspondingtime domain symbols within the principle portion of the time domainburst in order to compute a fine frequency offset, that is a fractionalcomponent of the frequency offset as measured in OFDM frequency domainsymbol widths. Once the fine offset is computed, it may be corrected byuse of appropriate control signals to the receiver variable frequencyoscillator. This procedure corrects for frequency offsets that are afraction of a frequency domain symbol width but after this correctionthe received frequency may still be offset from the transmit frequencyby an integer number of frequency domain symbol widths.

The procedure for correction of this integer frequency offset takesadvantage of frequency domain structure within each OFDM burst. EachOFDM burst includes regularly spaced training symbols having knownpredetermined values. The training symbols facilitate estimation of thechannel response at the receiver and correction of the integer frequencyoffset. The integer frequency offset is corrected by finding thefrequency alignment that causes the received symbol values at the knowntraining positions to correlate strongly between successive bursts.

It will be appreciated, however, that there is a limit to theacquisition range for this integer frequency offset correctiontechnique. If the frequency offset is greater than the spacing betweentraining symbols, then this offset correction technique may lock to afalse alignment that differs from the correct alignment by an integermultiple of the training symbol spacing. The acquisition range is thus$\pm \frac{N}{2\quad\upsilon}$tones where N is the number of frequency domain symbols in a single OFDMburst and v is the number of frequency domain symbols reserved fortraining.

Consider a millimeter wave application where the operating frequency is28 GHz and where use of low cost analog components may cause an initialfrequency offset of 10 parts per million (ppm) or 280 KHz. A typicalvalue, however, for $\frac{N}{2\quad\upsilon}$may be a small as 4 frequency domain symbol widths. In a representativesystem where the overall bandwidth of the OFDM system is 6 MHz and whereN is 256, this provides an acquisition range of only approximately ±94KHz or approximately ±3 ppm.

What is needed is a system for OFDM frequency synchronization that cancorrect for wide offsets that exceed the spacing between trainingsymbols within the frequency domain bursts.

SUMMARY OF THE INVENTION

Systems and methods for wide frequency offset synchronization areprovided by virtue of one embodiment of the present invention. Asynchronization data sequence is encoded onto a series of OFDM frequencydomain bursts as interburst phase differences between training symbolsincluded within the successive bursts. The interburst phase differencesmay also encode system configuration information. This technique may beused in conjunction with other synchronization techniques to greatlyextend the frequency acquisition range achievable with low cost analogcomponents.

A first aspect of the present invention provides a method forsynchronizing a second node to a first node in an OFDM communicationsystem. The method includes: at the second node, receiving a series offrequency domain bursts from the first node where the frequency domainbursts includes training symbols, measuring interburst phase differencesfor the training symbols, and determining a wide frequency offset basedon misalignment of a sequence of the interburst phase differencesrelative to a known sequence.

A second aspect of the present invention also provides a method ofsynchronizing a second node to a first node in an OFDM communicationsystem. The method includes: developing at the first node, a series offrequency domain bursts, where frequency domain bursts include trainingsymbols at a predetermined sequence of positions within the bursts, andincluding synchronization information in the frequency domain burstsencoded as a series of interburst phase differences for successivetraining symbol positions of the sequence, and transmitting thefrequency domain bursts to the second node.

Further understanding of the nature and advantages of the inventionherein may be realized by reference to the remaining portions of thespecification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a point to point communication system suitable forimplementing one embodiment of the present invention.

FIG. 2 depicts time domain structure of an OFDM burst as exploited byone embodiment of the present invention.

FIG. 3 depicts frequency domain structure of an OFDM burst as exploitedby one embodiment of the present invention.

FIG. 4 is a flowchart describing steps of synchronization according toone embodiment of the present invention.

FIG. 5 depicts a receiver according to one embodiment of the pre sentinvention.

FIG. 6 depicts a transmitter according to one embodiment of the presentinvention.

FIG. 7 is a flowchart describing steps of determining wide frequencyoffset according to one embodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 depicts a point to point OFDM communication system 100 suitablefor implementing one embodiment of the present invention. The presentinvention is, however, not limited in application to point to pointcommunication systems and may also be used in, e.g., point to multipointcommunications systems, broadcast systems, peer to peer systems, etc.System 100 includes a first node 102 and a second node 104. First node102 includes a transmitter 106 and a receiver 108. Transmitter 106 andreceiver 108 share use of an antenna 110 by employing a diplexer 112.Diplexer 112 is employed in FDD (frequency division duplexing) wheretransmitter 106 and receiver 108 can operate simultaneously but usingdifferent frequencies. The present invention also applies to TDD (timedivision duplexing) systems where transmitter 106 and receiver 108employ the same frequency but at different times. Diplexer 112 is thenunnecessary. A reference oscillator 114 provides a frequency standard toboth transmitter 106 and receiver 108.

Second node 104 includes a transmitter 116 and a receiver 118.Transmitter 116 and receiver 118 share use of an antenna 120 byemploying a diplexer 122. Instead of employing its own referenceoscillator, second node 104 synchronizes its operation frequency to thesignal received from first node 102. Receiver 118 synchronizes to itsreceived signal and generates a synchronization signal to control theoperation frequency of transmitter 116.

The frequency synchronization process described herein takes advantageof both time domain and frequency domain structure of OFDM burststransmitted from first node 102 to second node 104. FIG. 2 depicts atime domain structure 200 of an OFDM burst as exploited by oneembodiment of the present invention. Each burst includes a segmentholding N symbols representing a result of applying the Inverse FastFourier Transform (IFFT) to an N symbol frequency domain burst.Furthermore, each burst includes a v symbol cyclic prefix followed by anL symbol supplemental cyclic prefix. The N symbols of the IFFT resultfollow the cyclic prefixes. Together, the cyclic prefix and supplementalcyclic prefix replicate the last v+L symbols of the N symbol IFFTresult. The v symbol cyclic prefix assures reception of the frequencydomain symbols input into the IFFT within orthogonal subchannels even inthe face of dispersive channel conditions. The L symbols of thesupplemental cyclic prefix are used to facilitate synchronization.

FIG. 3 depicts a portion of an OFDM frequency domain structure 300exploited for synchronization purposes according to one embodiment ofthe present invention. The frequency domain structure includes v blocks,each block including one training symbol 302 and a group of N/v−1 datasymbols. Training symbols 302 have known magnitude values and are usedto estimate the response of the channel between first node 102 andsecond node 104. According to one embodiment of the present invention,training symbols 302 carry phase information useful to the receiver inestablishing synchronization and system configuration.

Frequency offset, as measured in frequency domain symbol widths, may beunderstood as including an integer portion and a fractional portion.f_(offset)=f_(int)+Δf_(offset) The synchronization process describedherein first corrects for the fractional portion of the frequency offsetand then corrects for the integer portion.

FIG. 5 depicts internal structure of receiver 118 of second node 104.Signals received via antenna 120 first go to a RF processing block 502where low noise preamplification and filtering occur. A mixer 504downconverts the processed RF signal to an intermediate frequency (IF)by mixing with the output of a variable frequency oscillator 506. Theoutput frequency of variable frequency oscillator 506 as set by afrequency control block 508 determines the receive frequency of receiver118. The IF signal is input into an IF processing block 510 whichfilters and amplifies at an IF frequency. There may be furtherdownconversion to baseband or downconversion may be inherent in theoperation of an analog to digital converter 512 which converts its inputsignal to a baseband series of complex symbol values. The basebandsymbols are input to an FIR filter 514. The output of FIR filter 514 isa series of time domain OFDM bursts.

FIG. 4 is a flowchart describing steps of a frequency synchronizationprocess according to one embodiment of the present invention. At step402, a fractional frequency offset processing block 516 evaluates a costfunction based on the fractional frequency offset. The cost function isevaluated using the following expression:${d(\delta)} = {\sum\limits_{k = {\delta - L + 1}}^{\delta}{{x^{*}(k)}{x\left( {k + N} \right)}}}$

where x(k) is a received time domain symbol value within structure 200,and where δ represents the position of the first of the N symbols ineach time domain OFDM burst as determined by a timing synchronizationprocess such as the one described in U.S. patent application Ser. No.09/245,168. The cost function will repeat every N+v+L samples. Thefractional frequency offset cost function should be averaged oversuccessive bursts by:${\overset{\_}{d}(\delta)} = {\sum\limits_{k = 0}^{K}{d\left( {\delta + {k\left( {N + v + L} \right)}} \right)}}$

The fractional frequency offset is then given by:${\Delta\quad f_{offset}} = {\frac{1}{2\pi\quad N}\tan^{- 1}\frac{{Im}{\overset{\_}{d}\left( \delta_{0} \right)}}{{Re}{\overset{\_}{d}\left( \delta_{0} \right)}}}$${{where}\quad\delta_{0}} = {\arg\quad{\min\limits_{\delta}{\overset{\_}{d}(\delta)}}}$

At step 404, frequency control block 508 adjusts the output frequency ofvariable frequency oscillator 506 to correct the fractional frequencyoffset determined in step 402. An integer offset may however remain.

The integer offset is determined by computing a correlation betweenfrequency domain symbol values of successive bursts that occupypositions reserved for training symbols. The magnitude of thecorrelation signal is used to determine small integer offset, i.e., acomponent of the integer offset that is less than the spacing betweentraining symbols in structure 300. Phase information encoded onto thetraining symbols is used to determine wide frequency offset, i.e., howmany groups of N/v symbols are in the integer offset.

An FFT block 520 removes the cyclic prefix from successive OFDM timedomain bursts output by FIR filter 514 and converts the bursts to thefrequency domain. An integer frequency offset processor 518 determinesthe small and large integer offsets based on the frequency domainsymbols output by FFT block 520.

The small integer offset is determined by first forming:${Y(n)} = {\sum\limits_{k = 0}^{K}{{X^{*}\left( {n,k} \right)}{X\left( {n,{k + 1}} \right)}}}$

where X(n,k) is the the received frequency domain value at frequencydomain symbol n and burst k and K is a number of successive bursts overwhich Y(n) is evaluated, e.g., 40.

A cost function, e_(j), is calculated over groups of N/v frequencydomain symbols by${e_{j} = {\sum\limits_{n \in J_{j}}{{Y(n)}\quad{w{here}}\quad{e.g}}}},{{{.\quad N}/v} = {{8\quad{and}\quad j} \in \left\lbrack {{- 4},3} \right\rbrack}}$

where J_(j) is the set of v frequency indices corresponding to thetraining symbol positions, and equally spaced by N/v.$J_{j} = \left\lbrack {{j\quad j} + {\frac{N}{\upsilon}\quad\ldots}}\quad \right\rbrack$

Small integer offset is determined based on a magnitude of a costfunction determined by correlating successive pairs of bursts.$f_{small} = {\arg{\max\limits_{j}{e_{j}}^{2}}}$

The determination of large frequency offset is dependent onsynchronization information encoded in the phase relationships betweentraining symbols. There are two sets of training symbols A and B thatare used in alternate bursts. Between each training symbol of burst Aand the corresponding training symbol of group B, there is a phasedifference Δφ or (D₀(n)). Alignment to a sequence derived from thepredefined sequence of D₀ values is determined at the receiver end toestimate large integer frequency offset. In addition to thesynchronization sequence, D₀ values may also encode system configurationinformation.

FIG. 6 depicts elements of transmitter 106 according to one embodimentof the present invention. A physical layer processor 702 coordinatesoverall transmitter operation, defines encoding and modulationparameters, and arranges for their transmission. Data to be transmittedis input to a Reed-Solomon encoder 704. Data output by Reed-Solomonencoder 704 is in the form of Reed-Solomon codewords. Each codewordincludes 2*t parity bytes where t is defined by a configuration signalfrom physical layer control processor 702.

Reed-Solomon codewords are forwarded to a byte interleaver 706. Byteinterleaver 706 reorders the encoded bytes to improve resistance toburst channel impairments. The time span over which byte interleaver 706reorders bytes is known as the interleaver depth and is controlled by aninterleaver depth signal generated by physical layer control processor702.

A convolutional encoder 708 applies a convolutional code. The degree ofredundancy introduced by convolutional encoder 708 may be varied byperiodic deletions of its output bits. Variation in the frequency ofdeletions implements a variation in the convolutional encoder rate,i.e., the ratio of input bits to output bits. The rate is controlled bya signal from physical layer control processor 702.

The output of convolutional encoder 708 is then input into a symbolmapper 710. Symbol mapper 710 maps bits to symbols in accordance with acurrently defined symbol constellation. Symbol mapper 710 may employ avariety of constellations. The currently employed constellation isdetermined by a constellation size control signal generated by physicallayer control processor 702. The output of symbol mapper 710 thenconsists of a stream of data symbols for inclusion within frequencydomain OFDM bursts.

Physical layer control processor 702 also outputs system configurationinformation to be encoded onto training symbols. In one embodiment, asingle byte indicates a current constellation size, convolutional coderate, interleaver depth, and number of Reed-Solomon parity bytes. Themapping between possible values of these parameters and byte valueswithin a single system configuration byte is presented in the followingtable:

Parameter Possible Values Bit Mapping Constellation Size 4 00XX XXXX 1601XX XXXX 64 10XX XXXX 256 11XX XXXX Code Rate ½ XX00 XXXX ⅔ XX01 XXXX ⅚XX10 XXXX ⅞ XX11 XXXX Interleaver Depth 4 XXXX 000X 6 XXXX 001X 9 XXXX010X 12 XXXX 011X 18 XXXX 100X 24 XXXX 101X 28 XXXX 110X 36 XXXX 111X RSParity 14 XXXX XXX0 20 XXXX XXX1

There are two sets of QPSK symbols that are used for the trainingsymbols. These two sets of training symbols, set A and set B, aredeveloped by training symbol symbol formation block 714 and sent inalternating OFDM bursts. Set A is selected to have a low Peak-to-MeanPower Ratio (PMPR). Set B is a modulated version of set A. Themodulation is based on a synchronization sequence and the configurationdata.

The synchronization sequence, p, is a binary, maximal length sequence oflength v. This binary sequence defines a set of 0 or 90 degree phaseshifts in set A. That is, if p(k)=0, C(k)=A(k), and if p(k)=1,C(k)=A(k)*exp(j*π/2).

The data sequence is formed from the 8 bits of configuration datadescribed in system configuration byte. These data bits aredifferentially encoded to form sequence d, and mapped to 0 or 180 degreephase shifts, and then applied to C(k) to form B(k). Thus, B(k)=C(k) for0<k<v−8; B(i+v−8)=C(i+v−8) for i=0 . . . 7, p(i)=0, andB(i+v−8)=−C(i+v−8) for i=0 . . . 7,p(i)=1.

The output of differential coding block 716 consists of sets of trainingsymbols for inclusion within bursts of type A and B. The sets oftraining symbols are output in an alternating pattern, i.e., bursts k,k+2, k+4 . . . use set A while bursts k+1, k+3, . . . use set B.

A selection block 718 then forms successive bursts of type A and B bycombining the frequency domain data symbols output by symbol mapper 710and the training symbols output by differential coding block 716. In oneembodiment, the training symbols are evenly spaced through the burst.Selection block 718 forms the bursts and outputs successive frequencydomain OFDM bursts to an IFFT block 720. IFFT block 720 converts thefrequency domain burst to the time domain and affixes cyclic prefixes. Atransmitter system 722 converts the baseband digital signal to analog,upconverts the signal to an intermediate frequency (IF), amplifies andotherwise processes the IF signal, upconverts the IF signal to a radiofrequency (RF), amplifies and otherwise processes the RF signal, andtransmits the RF signal via an antenna 724.

There is a synchronization sequence of interspersed phase differencesfor the training symbols described by the expression:D ₀ =Z* _(A)(n)Z _(B)(n)∀n∈1 . . . v,

Sequence alignment is based on the differences between successive D₀values from training symbol position to training symbol position. Tofacilitate the determination of the large integer frequency offset,integer frequency offset processor 518 maintains not only the D₀ valuesbut also a series of D₁ values representing these position to positionphase differences. The D₁ sequence is derived from the D₀ sequence by:D ₁(n)=D ₀(n)D ₀*(n+1)∀n∈1 . . . v,where D₀(1)=D₀(v)A sequence of D₂ values is also maintained as follows:D ₂(n)=|Re(D ₁(n))|+i|Im(D ₁(n))|∀n∈1 . . . vEstimates of the D₀, D₁, D₂ sequences are formed by integer frequencyoffset processor 518 over K bursts using the received training tonevalues T₁, FIG. 7 is a flowchart describing steps of determining largeinteger frequency offset according to one embodiment of the presentinvention. At step 802, the D₀ values are estimated for each burst k by:{circumflex over (D)} ₀(n,k)=X*(n,k)X(n,k+1)∀n∈1 . . . v, and ∀kAt step 804, the D₁ values are estimated by:${{\hat{D}}_{1}(n)} = {\frac{1}{K - 1}{\sum\limits_{k = 1}^{K - 1}{{{\hat{D}}_{0}\left( {n,k} \right)}{{\hat{D}}_{0}\left( {{n + 1},k} \right)}}}}$where {circumflex over (D)}₀(1)={circumflex over (D)}₀(v)As can be seen, the D₁ values are smoothed over bursts.

The D₂ values are then estimated based on:D̂₂(n) = Re(D̂₁(n) + iIm(D̂₁(n))∀n ∈ 1  …  v,The large integer frequency offset is determined by calculating costsfor each possible alignment of the received estimate D₂ values to the D₂values derived from the known D₀ sequence. The cost function is:${{dl}(r)} = {\sum\limits_{M = 0}^{\upsilon - 1}{{{\hat{D}}_{2}\left( {M + r} \right)}{D_{2}(M)}}}$where r=[−v/2 . . . v/2]At step 806, the large integer offset is determined based on the maximumvalue of dl(r) as follows: $\begin{matrix}{{dl}_{\max} = {\arg{\max\limits_{r}{{dl}(r)}}}} \\{f_{large} = {\frac{N}{\upsilon}{dl}_{\max}}}\end{matrix}$

At step 808, system configuration data is retrieved from the last 8training symbols. First, the order of the alternating training symbolssets A and B must be determined. This order can be determined byfinding:${\sum\limits_{{neJ}_{config}}{{{Im}\left( {{\hat{D}}_{1}\left( {n = {dl}_{\max}} \right)} \right)}{{Im}\left( {D_{1}(n)} \right)}}} > 0$where J_(config) is the set of training symbols carrying configurationinformation. If this inequality is true than A precedes B while if theinequality is not true then B precedes A. After the order of thealternating training symbol sets is determined, the values of the bitsof the system configuration byte can then be determined.

For each of the positions in J_(config), the configuration data can befound by comparing the phase of D₁ to the known synchronizationsequence, C(K). If D₁(K) is within 90 degrees of C(K), then theconfiguration data for the corresponding symbol is 0, otherwise the datais 1.

The total integer frequency offset is then the total of the largeinteger and small integer offsets. Referring again to FIG. 4, at step412, frequency control block 508 adjusts the operating frequency ofvariable frequency oscillator 506 to correct for both the large andsmall integer offsets as determined by integer frequency offsetprocessing block 518. The receive frequency of receiver 118 is thenaligned to the transmit frequency of transmitter 106. The output ofvariable frequency oscillator 506 can then serve as a frequencysynchronization signal to transmitter 116 so that transmitter 116'stransmission frequency will then be locked to the transmission frequencyof transmitter 106. A signal processing block 522 performs furthersignal processing on the frequency domain training and data symbols torecover transmitted data. This processing includes estimation of thechannel response and correction of the received data symbols for theestimated channel response.

It is understood that the examples and embodiments described herein arefor illustrative purposes and that various modifications or changes inlight thereof will be suggested to persons skilled in the art and are tobe included within the spirit and purview of this application and scopeof the appended claims and their full scope of equivalents. For example,the present invention may be applied to wired systems rather thanwireless systems. Also, it will be appreciated that the presentinvention may be applied to receiver systems that incorporate input frommultiple antennas. All publications, patents and patent applicationscited herein are hereby incorporated by reference.

1. In an OFDM communication system, a method for synchronizing a secondnode to a first node, the method comprising: at said second node,receiving a series of frequency domain bursts from said first node, saidfrequency domain bursts including training symbols and data symbols assubcarriers within the same burst; measuring interburst phasedifferences for said training symbols; determining a wide frequencyoffset based on misalignment of a sequence of said interburst phasedifferences relative to a known sequence; and determining systemconfiguration information based on said interburst phase differenceswherein said system configuration information comprises at least one ofconstellation size, code rate, interleaver depth, and RS parity.
 2. Themethod of claim 1 further comprising: using said training symbols toestimate a channel response.
 3. The method of claim 1 furthercomprising: adjusting receiver frequency responsive to said widefrequency offset.
 4. The method of claim 1 wherein said sequence ofinterburst phase differences includes interburst phase differences forparticular training symbol positions.
 5. In an OFDM communicationsystem, a method of synchronizing a second node to a first node, saidmethod comprising: developing at said first node, a series of frequencydomain bursts, said frequency domain bursts including training symbolsat a predetermined sequence of subcarrier positions within said burstsand also data symbols at other subcarrier positions; includingsynchronization information in said frequency domain bursts encoded as aseries of interburst phase differences for successive training symbolpositions of said sequence; and transmitting said frequency domainbursts to the second node; and wherein at least one of said series ofinterburst phase differences further encodes system configurationinformation, said system configuration information comprising at leastone of constellation size, code rate, interleaver depth, and RS parity.6. The method of claim 5 further comprising: converting said frequencydomain bursts to time domain bursts.
 7. The method of claim 5 whereinsaid training symbols are evenly spaced within said frequency domainbursts.
 8. In an OFDM communication system, apparatus for synchronizinga second node to a first node, said apparatus comprising: a system thatreceives a series of frequency domain bursts from said first node, saidfrequency domain bursts including training symbols and data symbols assubcarriers; a frequency offset processor that measures interburst phasedifferences for said training symbols, and that determines a widefrequency offset based on misalignment of a sequence of said interburstphase differences relative to a known sequence; and a systemconfiguration processor that determines system configuration informationbased on said interburst phase differences, said system configurationinformation comprising at least one of constellation size, code rate,interleaver depth, and RS parity.
 9. The apparatus of claim 8 furthercomprising: a channel estimation processor that uses said trainingsymbols to estimate a channel response.
 10. The apparatus of claim 8further comprising: a frequency control block that adjusts receiverfrequency responsive to said wide frequency offset.
 11. The apparatus ofclaim 8 wherein said sequence of interburst phase differences includesinterburst phase differences for particular training symbol positions.12. In an OFDM communication system, apparatus for synchronizing asecond node to a first node, said apparatus comprising: a trainingsymbol development system that develops at said first node, a series offrequency domain bursts, said frequency domain bursts including trainingsymbols at a predetermined sequence of subcarrier positions within saidbursts and data symbols at other subcarrier positions within saidbursts; and a synchronization sequence generation system that includessynchronization information in said frequency domain bursts encoded as aseries of interburst phase differences for successive training symbolpositions of said sequence; and wherein at least one of said series ofinterburst phase differences further encodes system configurationinformation, said system configuration information comprising at leastone of constellation size, code rate, interleaver depth, and RS parity.13. The apparatus of claim 12 further comprising: a transform block thatconverts said frequency domain bursts to time domain bursts.
 14. Theapparatus of claim 12 wherein said training symbols are evenly spacedthrough said frequency domain bursts.
 15. In an OFDM communicationsystem, apparatus for synchronizing a second node to a first node, saidapparatus comprising: means for receiving a series of frequency domainbursts from said first node, said frequency domain bursts includingtraining symbols and data symbols at within the same burst; and meansfor measuring interburst phase differences for said training symbols;means for determining a wide frequency offset based on misalignment of asequence of said interburst phase differences relative to a knownsequence; and means for determining system configuration informationbased on said interburst phase differences, said system configurationinformation comprising at least one of constellation size, code rate,interleaver depth, and RS parity.
 16. The apparatus of claim 15 furthercomprising: means for estimating a channel response using said trainingsymbols.